To do it the "true" way --
There's a tutorial on Youtube here:
https://www.youtube.com/watch?v=wTeSwmpHLucIt's not a fun process. Basically once you get the model working (as shown in the video), you need to copy all the code from one moveset file to the other. Also you obviously can't use articles from that previous character but you can use articles from the new character.
To do it the rel way --
I've never done this but here's a tutorial on youtube (that I've never watched):
https://www.youtube.com/watch?v=y8yi-JI_GnMTo do it the "ehhhhh" way --
Just stick everything over the other character with no changes. Like literally just rename the files and see what happens. Sometimes it works and there will be minor glitches that you can just buff out in PSA -- some can't be fixed. I don't recommend this but ehhhh it does the job sometimes. If it freezes, try doing the model editing shown in the video posted under the "true" way and give it another go. I've had success doing this method in the past by putting things over Marth and Ike. I forget much about doing it this way though, because I now only do it the "true" way for lasting stability, but ehhhhhhhhhhh.